The present invention relates to a dot line printer employed as an output device in a personal computer or the like.
As the head carriage is driven in a dot line printer, a plurality of printing heads arranged on the head carriage at equal intervals are moved with a predetermined stroke in a direction perpendicular to the sheet feeding direction to print on a printing sheet set on the platen. In order to allow the printing heads to print in the desired place on the printing sheet, it is essential to detect the printing time for the printing heads with high accuracy.
A conventional dot line printer having a head carriage driving mechanism and a printing timing detecting mechanism is shown in FIG. 1.
In FIG. 1, reference numeral 1 designates a head carriage. The head carriage 1 has a plurality of printing heads 2 arranged at equal intervals. The printing heads 2 confront a printing sheet A on a platen 3. The arrow 100 on printing sheet A indicates the direction of movement of printing sheet A relative to printing heads 2. A head carriage driving motor 5 is provided at a predetermined position on a frame 4. A worm gear 6 is mounted on the shaft 5a of the motor 5. The worm gear 6 is engaged with a worm wheel 7 provided on the frame 4. The worm wheel 7 is engaged with a gear 8 which is supported on the frame 4. A rotary encoder 9 is mounted on the shaft 8a which supports the gear 8. The rotary encoder 9 is made up a rotary disk 9a, coaxial with the gear 8, and has elongated slits 9b radially arrange about the circumference of disk 9a, and a photo sensor 9c. Photo sensor 9c is positioned relative to rotary disk 9a to detect slits 9b in rotary disk 9a as rotary disk 9a rotates. A cam 10 for moving the head carriage 1 linearly and laterally for reciprocal movement is provided on the rotary encoder 9. The cam 10 and corresponding cam post 10b define a track therebetween designated as cam groove 10a. Cam follower 11 has one end slidably engaged in cam groove 10a. The other end of cam follower 11 is secured to carriage 1.
When the motor 5 is driven in a direction indicated by arrow 101, the rotation of the motor 5 is transmitted through the worm gear 6, the worm wheel 7, which rotates as shown by arrow 102, and the gear 8 to the cam 10, which rotates as shown by arrow 103, to thereby drive cam follower 11 and head carriage 1. As a result, the head carriage 1 is moved linearly and laterally at a uniform speed in a direction perpendicular to the sheet feeding direction as indicated by arrow 100. In this operation, the printing timing of the printing heads 2 is detected as photo sensor 9c counts the slits 9b in the rotating rotary disk 9a, thereby detecting the number of revolutions of rotational angle of the rotary disk 9a.
In dot line printers of this type it is essential to detect the printing timing accurately. In the above-described conventional dot line printer, it is essential to form the slits 9b in the rotary disk 9a with high precision to insure accurate printing timing. However, the manufacturing cost, particularly for the precision manufacture of disk 9a, is high, and the detecting mechanism is expensive. The device also is disadvantageous in that, if the cam 10 is deformed, the rotary encoder 9 must be replaced in its entirety. In addition, since the cam follower 11 is slidably tracked within cam groove 10a, the cam 10, cam post 10b and/or the cam follower 11 rapidly become worn during use thereby altering the stroke of the head carriage out of alignment. As a result, it becomes impossible to detect the printing timing accurately, and to print in place.